The existence of electricity, the phenomenon
associated with stationary or moving electric charges, has been known
since the Greeks discovered that amber, rubbed with fur, attracted light
objects such as feathers. Ben Franklin proved the electrical nature of
lightning (the famous key experiment) and also established the
conventional use of negative and positive types of charges.

Coulomb quantifies amount of charge and discovers force law
between charges

electrical force is similar to gravity in that it is inverse proportional to the square of the distance
between charges

charges are assigned positive or negative values

like charges repel, opposites attract

It was also known that certain materials blocked electric charge, called insulators,
such as glass or cork. Other materials transfered electric charge with ease, called
conductors, such as metal.

By the 18th century, physicist Charles Coulomb defined the quantity of
electricity later known as a coulomb, and determined the force law between
electric charges, known as Coulomb's law. Coulomb's law is similar to the
law of gravity in that the electrical force is inversely proportional to
the distance of the charges squared, and proportional to the product of
the charges.

By the end of the 18th century, we had determined that electric charge
could be stored in a conducting body if it is insulated from its
surroundings. The first of these devices was the Leyden jar. consisted
of a glass vial, partly filled with sheets of metal foil, the top of which
was closed by a cork pierced with a wire or nail. To charge the jar, the
exposed end of the wire is brought in contact with a friction device.

modern atomic theory explains the origin of electric charge to the existence of small negative particles
(electrons) and small positive particles (protons)

an atom can gain electrons in its outer shells to gain negative charge or

lose electrons increasing the strength of the proton charge (positive)

while each atomic charge is small, the number of atoms in a typical macroscopic piece of matter is very large

thus, electric power becomes the primary source of energy for civilization

Modern atomic theory explains this as the ability of atoms to
either lose or gain an outer electron and thus exhibit a net positive
charge or gain a net negative charge (since the electron is negative).
Today we know that the basic quantity of electric charge is the electron,
and one coulomb is about 6.24x1018 electrons.

The battery was invented in the 19th century, and electric current and
static electricity were shown to be manifestations of the same
phenomenon, i.e. current is the motion of electric charge. Once a laboratory
curiosity, electricity becomes the focus of industrial concerns when it is shown
that electrical power can be transmitted efficiently from place to place and with
the invention of the incandescent lamp.

Electric Fields:

the mathematical problem of describing motion of electric charges leads to the development of the field
concept

the use of fields (as in electric field, gravitational field, force field) is mathematical tool to describe
physical events

the field itself is not observable, only its effects (motion)

The discovery of Coulomb's law, and the behavior or motion of charged
particles near other charged particles led to the development of the
electric field concept.

A field can be considered a
type of energy in space, or energy with position. A field is usually visualized as
a set of lines surrounding the body, however these lines do not exist, they are
strictly a mathematical construct to describe motion. Fields are used in
electricity, magnetism, gravity and almost all aspects of modern physics.

the shape, and resulting strength, of an electric field is dependent on the number and position of the
electric charges

each point in space can be assigned a vector to determine the direction and magnitude of motion

An electric field is the region around an electric charge in which an
electric force is exerted on another charge. Instead of considering the
electric force as a direct interaction of two electric charges at a
distance from each other, one charge is considered the source of an
electric field that extends outward into the surrounding space, and the
force exerted on a second charge in this space is considered as a direct
interaction between the electric field and the second charge.

Magnetism:

related to electricity, is the phenomenon of magnetism

magnetism results from the motion of electric charges, although historically we know of the behavior of
magnetic fields from bar magnets

magnetic fields deflect moving charges or other magnets

magnets have north and south poles, similar to positive and negative charge, but poles can
never be separated

Magnetism is the phenomenon associated with the motion of electric
charges, although the study of magnets was very confused before the
19th century because of the existence of ferromagnets, substances such as
iron bar magnets which maintain a magnetic field where no obvious electric
current is present (see below). Basic magnetism is the existence of magnetic fields
which deflect moving charges or other magnets. Similar to electric force in
strength and direction, magnetic objects are said to have `poles' (north and south,
instead of positive and negative charge). However, magnetic objects are always
found in pairs, there do not exist isolated poles in Nature.

the simplest magnetic field results from an electric current circulating in a loop

bar, or permanent, magnets result from small atomic dipole loops which sum to produce a macroscopic magnetic
field

The most common source of a magnetic field is an electric current loop. The motion
of electric charges in a pattern produces a magnetic field and its associated
magnetic force. Similarly, spinning objects, like the Earth, produce magnetic
fields, sufficient to deflect compass needles.

Today we know that permanent magnets are due to dipole charges inside the magnet at
the atomic level. A dipole charge occurs from the spin of the electron around the
nucleus of the atom. Materials (such as metals) which have incomplete electron shells
will have a net magnetic moment. If the material has a highly ordered crystalline
pattern (such as iron or nickel), then the local magnetic fields of the atoms become
coupled and the material displays a large scale bar magnet behavior.

Electromagnetism:

in a key development for modern physics, electricity and magnetism were `unified' into electromagnetism

the connection develops from the fact that an electric current (the flow of electrons in a metal) produces a
magnetic field

Faraday shows that a changing electric field produces a magnetic field and, vice-versus, a changing magnetic
field produces an electric current

Maxwell completes the theory with a full mathematical description of the relationship between electric and
magnetic fields = electromagnetism

A connection between electricity and magnetism had long been suspected,
and in 1820 the Danish physicist Hans Christian Orsted showed that an
electric current flowing in a wire produces its own magnetic field.
Andre-Marie Ampere of France immediately repeated Orsted's experiments and
within weeks was able to express the magnetic forces between
current-carrying conductors in a simple and elegant mathematical form. He
also demonstrated that a current flowing in a loop of wire produces a
magnetic dipole indistinguishable at a distance from that produced by a
small permanent magnet; this led Ampere to suggest that magnetism is caused
by currents circulating on a molecular scale, an idea remarkably near the
modern understanding.

Faraday, in the early 1800's,
showed that a changing electric field produces a magnetic field, and that
vice-versus, a changing magnetic field produces an electric current. An
electromagnet is an iron core which enhances the magnetic field generated
by a current flowing through a coil, was invented by William Sturgeon in
England during the mid-1820s. It later became a vital component of both
motors and generators.

The unification of electric and magnetic phenomena in a complete
mathematical theory was the achievement of the Scottish physicist Maxwell (1850's). In a set of four
elegant equations, Maxwell formalized the relationship between electric and
magnetic fields. In addition, he showed that a linear magnetic and
electric field can be self-reinforcing and must move at a particular
velocity, the speed of light. Thus, he concluded that light is energy
carried in the form of opposite but supporting electric and magnetic
fields in the shape of waves, i.e. self-propagating electromagnetic
waves.

Maxwell's new theory provides a new description of light, as electromagnetic waves

electromagnetism represents a sharp change in the way Nature is described, i.e. the use of invisible fields
and understanding that can only be communicated with mathematics

In doing this, Maxwell moved physics to a new realm of understanding. By
using field theory as the core to electromagnetism, we have moved beyond a
Newtonian worldview where objects change by direct contact and into a
theory that uses invisible fields. This introduces a type of
understanding which can only be described with a type of mathematics that
cannot be directly translated into language. In other words, scientists
where restricted in talking about electromagnetic phenomenon strictly
through the use of a new type of language, one of pure math.

Electromagnetic Radiation (a.k.a. Light):

light is electromagnetic radiation propagating through space

the wavelength of the light determines its energy and characteristics

Electromagnetic radiation is energy that is propagated through free
space or through a material medium in the form of electromagnetic
waves, such as radio waves, visible light, and gamma rays. The term
also refers to the emission and transmission of such radiant energy.

The wavelength of the light determines its characteristics. For example, short
wavelengths are high energy gamma-rays and x-rays, long wavelengths are radio
waves. The whole range of wavelengths is called the electromagnetic
spectrum.

In 1887 Heinrich Hertz, a German physicist, provided
experimental confirmation of Maxell's ideas by producing the first man-made
electromagnetic waves and investigating their properties. Subsequent
studies resulted in a broader understanding of the nature and origin
of radiant energy.

light always travels at the same speed in a vacuum, 299,792 meters per second

It has been established that time-varying electric fields can induce
magnetic fields and that time-varying magnetic fields can in like
manner induce electric fields. Because such electric and magnetic
fields generate each other, they occur jointly, and together they
propagate as electromagnetic waves. An electromagnetic wave is a
transverse wave in that the electric field and the magnetic field at
any point and time in the wave are perpendicular to each other as
well as to the direction of propagation. In free space (i.e., a space
that is absolutely devoid of matter and that experiences no intrusion
from other fields or forces), electromagnetic waves always propagate
with the same speed--that of light (299,792,458 m per second, or
186,282 miles per second)--independent of the speed of the observer
or of the source of the waves.

Electromagnetic radiation has properties in common with other forms
of waves such as reflection, refraction, diffraction, and
interference. Moreover, it may be characterized by the frequency with
which it varies over time or by its wavelength. Electromagnetic
radiation, however, has particle-like properties in addition to those
associated with wave motion. It is quantized in that for a given
frequency , its energy occurs as an integer times h , in which h is a
fundamental constant of nature known as Planck's constant. A quantum
of electromagnetic energy is called a photon. Visible light and other
forms of electromagnetic radiation may be thought of as a stream of
photons, with photon energy directly proportional to frequency.

Electromagnetic radiation spans an enormous range of frequencies or
wavelengths, as is shown by the electromagnetic spectrum.
Customarily, it is designated by fields, waves, and particles in
increasing magnitude of frequencies--radio waves, microwaves,
infrared rays, visible light, ultraviolet light, X rays, and gamma
rays. The corresponding wavelengths are inversely proportional, and
both the frequency and wavelength scales are logarithmic.

various wavelengths are absorbed by the Earth's atmosphere (for example, to protect us from damaging high
energy light)

different types of astronomy questions require different wavelengths = different technology

Electromagnetic radiation of different frequencies interacts with
matter differently. A vacuum is the only perfectly transparent
medium, and all material media absorb strongly some regions of the
electromagnetic spectrum. For example, molecular oxygen
(O2), ozone (O3), and molecular nitrogen (N2) in the
Earth's atmosphere are almost perfectly transparent to infrared rays
of all frequencies, but they strongly absorb ultraviolet light, X
rays, and gamma rays. The frequency (or energy equal to hv) of X rays
is substantially higher than that of visible light, and so X rays are
able to penetrate many materials that do not transmit light.
Moreover, absorption of X rays by a molecular system can cause
chemical reactions to occur. When X rays are absorbed in a gas, for
instance, they eject photoelectrons from the gas, which in turn
ionize its molecules. If these processes occur in living tissue, the
photoelectrons emitted from the organic molecules destroy the cells
of the tissue. Gamma rays, though generally of somewhat higher
frequency than X rays, have basically the same nature. When the
energy of gamma rays is absorbed in matter, its effect is virtually
indistinguishable from the effect produced by X rays.

There are many sources of electromagnetic radiation, both natural and
man-made. Radio waves, for example, are produced by cosmic objects
such as pulsars and quasars and by electronic circuits. Sources of
ultraviolet radiation include mercury vapor lamps and high-intensity
lights, as well as the Sun. The latter also generates X rays, as do
certain types of particle accelerators and electronic devices.

Wave Properties:

Due to its wave-like nature, light has three properties when encountering
a medium:

Reflection is the abrupt change in the direction of propagation of a
wave that strikes the boundary between different mediums. At least
part of the oncoming wave disturbance remains in the same medium.
Regular reflection, which follows a simple law, occurs at plane
boundaries. The angle between the direction of motion of the
oncoming wave and a perpendicular to the reflecting surface (angle of
incidence) is equal to the angle between the direction of motion of
the reflected wave and a perpendicular (angle of reflection).
Reflection at rough, or irregular, boundaries is diffuse. The
reflectivity of a surface material is the fraction of energy of the
oncoming wave that is reflected by it.

Refraction is the change in direction of a wave passing from one
medium to another caused by its change in speed. For example, waves
in deep water travel faster than in shallow; if an ocean wave
approaches a beach obliquely, the part of the wave farther from the
beach will move faster than that closer in, and so the wave will
swing around until it moves in a direction perpendicular to the
shoreline. The speed of sound waves is greater in warm air than in
cold; at night, air is cooled at the surface of a lake, and any sound
that travels upward is refracted down by the higher layers of air
that still remain warm. Thus, sounds, such as voices and music, can
be heard much farther across water at night than in the daytime.

The electromagnetic waves constituting light are refracted when
crossing the boundary from one transparent medium to another because
of their change in speed. A straight stick appears bent when partly
immersed in water and viewed at an angle to the surface other than
90. A ray of light of one wavelength, or color (different
wavelengths appear as different colors to the human eye), in passing
from air to glass is refracted, or bent, by an amount that depends on
its speed in air and glass, the two speeds depending on the
wavelength. A ray of sunlight is composed of many wavelengths that in
combination appear to be colorless; upon entering a glass prism, the
different refractions of the various wavelengths spread them apart as
in a rainbow.

Diffraction is the spreading of waves around obstacles. Diffraction
takes place with sound; with electromagnetic radiation, such as
light, X-rays, and gamma rays; and with very small moving particles
such as atoms, neutrons, and electrons, which show wavelike
properties. One consequence of diffraction is that sharp shadows are
not produced. The phenomenon is the result of interference (i.e.,
when waves are superimposed, they may reinforce or cancel each other
out) and is most pronounced when the wavelength of the radiation is
comparable to the linear dimensions of the obstacle. When sound of
various wavelengths or frequencies is emitted from a loudspeaker, the
loudspeaker itself acts as an obstacle and casts a shadow to its rear
so that only the longer bass notes are diffracted there. When a beam
of light falls on the edge of an object, it will not continue in a
straight line but will be slightly bent by the contact, causing a
blur at the edge of the shadow of the object; the amount of bending
will be proportional to the wavelength. When a stream of fast
particles impinges on the atoms of a crystal, their paths are bent
into a regular pattern, which can be recorded by directing the
diffracted beam onto a photographic film.

The angle of refraction is greater for a denser medium and is also a
function of wavelength (i.e. blue light is more refracted compared to
red and this is the origin to rainbows from drops of water)

Diffraction is the constructive and destructive interference of two
beams of light that results in a wave-like pattern

Absorption spectrum - a continuous spectrum that passes through a cool
gas has specific spectral lines removed (inverse of an emission
spectrum)

Planck's curve:

One of the primary results from the field of spectroscopy was the
discovery of how the energy outputed by an object (its spectrum) changes
with temperature

Stefan-Boltzmann law: the amount of energy emitted from a body
increases with higher temperature

Wien's law: the peak of emission moves to bluer light as
temperature increases

the energy outputed by an object (a star, a piece of metal, a human body) takes on a
particular shape called Planck's curve, shown in the following plot of energy versus
wavelength

notice that all objects emit all kinds of electromagnetic radiation. Except, cool
objects (like humans) emit very little at short wavelengths (x-rays) and long
wavelengths (radio). Most of our energy comes out in the infrared (our peak
emission is at 10 microns)

The quantum theory of absorption and emission of radiation announced
in 1900 by Planck ushered in the era of modern physics. He proposed
that all material systems can absorb or give off electromagnetic
radiation only in "chunks" of energy, quanta E, and that these are
proportional to the frequency of that radiation E = h. (The constant
of proportionality h is, as noted above, called Planck's constant.)

Planck was led to this radically new insight by trying to explain the
puzzling observation of the amount of electromagnetic radiation
emitted by a hot body and, in particular, the dependence of the
intensity of this incandescent radiation on temperature and on
frequency. The quantitative aspects of the incandescent radiation
constitute the radiation laws.

The Austrian physicist Josef Stefan found in 1879 that the total
radiation energy per unit time emitted by a heated surface per unit
area increases as the fourth power of its absolute temperature T
(Kelvin scale). This means that the Sun's surface, which is at T =
6,000 K, radiates per unit area (6,000/300)4 = 204 = 160,000 times
more electromagnetic energy than does the same area of the Earth's
surface, which is taken to be T = 300 K. In 1889 another Austrian
physicist, Ludwig Boltzmann, used the second law of thermodynamics to
derive this temperature dependence for an ideal substance that emits
and absorbs all frequencies. Such an object that absorbs light of
all colors looks black, and so was called a blackbody.

The wavelength or frequency distribution of blackbody radiation was
studied in the 1890s by Wilhelm Wien of Germany. It was his idea to
use as a good approximation for the ideal blackbody an oven with a
small hole. Any radiation that enters the small hole is scattered and
reflected from the inner walls of the oven so often that nearly all
incoming radiation is absorbed and the chance of some of it finding
its way out of the hole again can be made exceedingly small. The
radiation coming out of this hole is then very close to the
equilibrium blackbody electromagnetic radiation corresponding to the
oven temperature. Wien found that the radiative energy dW per
wavelength interval d has a maximum at a certain wavelength m and
that the maximum shifts to shorter wavelengths as the temperature T
is increased, as illustrated in the figure below.

Wien's law of the shift of the radiative power maximum to higher
frequencies as the temperature is raised expresses in a quantitative
form commonplace observations. Warm objects emit infrared radiation,
which is felt by the skin; near T = 950 K a dull red glow can be
observed; and the color brightens to orange and yellow as the
temperature is raised. The tungsten filament of a light bulb is T =
2,500 K hot and emits bright light, yet the peak of its spectrum is
still in the infrared according to Wien's law. The peak shifts to the
visible yellow when the temperature is T = 6,000 K, like that of the
Sun's surface.